Ultrasonic non-destructive testing is one of the most important technologies to test materials and is able to detect flaws at the surface as well as inside of a material.
The surface deformations to be sensed occur in the range of frequencies extending from 0.5 MHz to 50 MHz at most. High ultrasonic frequencies are generally strongly attenuated by commonly used materials so that the range generally does not extend beyond 10 or 15 MHz. The displacements are generally much less than an optical visible wavelength (about 5000 .ANG.) and range from a fraction of 1 .ANG. to a few hundred .ANG. at most. If the surface is given a velocity v(t)(=d.delta.(t)/dt, where .delta.(t) is displacement), the Doppler shift is .DELTA.f(t)=2v(t)/.lambda., where .lambda. is the optical wavelength. Taking a peak displacement amplitude U=20 .ANG. at a frequency of f.sub.u =2.5 MHz (.delta.(t)=U cos 2.pi.f.sub.u t) gives a peak velocity 2.pi.f.sub.u U of about 0.03 m/s and a peak Doppler shift of about 120 KHz (2 parts in 10.sup.10 of the optical frequency). This order of magnitude shows that the velocities and relative Doppler shift are small and a very sensitive discriminator is needed to measure the surface deformations.
There have been, in the past, many interferometric systems which detect Doppler shifted radiation by ultrasonic waves and other moving targets. U.S. Pat. No. 3,355,934 issued on Dec. 5, 1967, (Foster), describes a non-contact vibration measurement system which uses a laser beam and a light frequency discriminator. The patent, however, fails to teach any specifics of the light frequency discriminator. Only description on the light frequency discriminator is found at column 5, line 13, et seq. of the patent, "Detector-discriminator can be used to put out a signal as the detected light frequency varies (similar to a standard FM discriminator) and thus measure velocity of motion of the device under study . . . ".
Michelson interferometers also have been widely used in the past for detecting Doppler shifted radiation. U.S. Pat. No. 4,046,477, Sept. 6, 1977, (Kaule), teaches an instrument of this type. This instrument, and other similar systems, detect and analyze the scattered beam from a material by using a Michelson optical interferometer. However, they have a very small light gathering efficiency when the surface being observed is not mirror-like. The reason for this is that a long optical path difference is needed for a Michelson interferometer to have adequate frequency discriminatory sensitivity and thus the central fringes of the interference are viewed under a very small angle.
The Fabry-Perot type interferometers have been studied in Review of Scientific Instruments, Vol. 39, No. 8, pp 1100-1103, August 1968, "Free surface velocity measurement of an impacted projectile by optical Doppler shift", by P. M. Johnson et al, and in Journal of Physics E: Scientific Instruments, Vol. 4, pp 170-172, 1977, "Rapid velocity sensor using a static confocal Fabry-Perot and a single frequency argon laser", by D. M. Paul et al.
U.S. Pat. No. 4,129,041, Dec. 12, 1978 (Bickel), makes mention of a Fabry-Perot interferometer to detect the Doppler shift. This patent describes mainly the use of the light absorption phenomenon to produce a filter giving the light frequency discriminator feature over a very wide acceptance angle. This property is valid for systems based on absorption by a gas, a liquid or a solid medium, since the filtering effect depends only upon the path travelled by light in the medium. The Fabry-Perot interferometer is assimilated wrongly to such filters. The acceptance angle of a Fabry-Perot interferometer is much more limited, especially in the case of the planar type which is considered in this patent. As a matter of fact, the planar Fabry-Perot has the same acceptance angle and the same etendue as the Michelson interferometer of U.S. Pat. No. 4,046,477 referred to above.
The light gathering efficiency is called "etendue" of a discriminating system and is equal to the area of its entrance aperture multiplied by the solid angle of the cone of the limit rays.
A Fabry-Perot interferometer of the confocal type (which is made of two concave mirrors having a common focal point) has a much larger etendue than the planar type.